1. Field of the Invention
This invention relates to negative electron beam resists for photomask fabrication and for semiconductor device fabrication.
2. Description of the Prior Art
A resist is an adhering layer of a material with patterned openings on a support which is used as a mask for etching, either partially or completely through, the support exposed by the openings in the resist. The use of light as the irradiator or energy source for fabricating photoresists in the semiconductor art has been common for many years. The photoresist method of semiconductor manufacture was adequate until the advent of small geometry high frequency devices and integrated circuits requiring the formation of patterns with line widths in the neighborhood of 1 micron. Although 1 micron line openings, or resolution, can be obtained from photoresists in the laboratory, such line widths are not reproducible due to diffraction problems, with the practical limit of production produced openings being in the neighborhood of 5 to 6 microns in width.
The step from the use of light to electrons to form resists was a logical one. Theoretically, since the size of an electron is only 1/1000th the size of a quantum of light, an electron beam should produce openings with line widths 1/1000th the size of openings obtained with photoresists. However, due to electron bounce-back or back scatter from the surface supporting the resist, such small width openings are not obtainable, only 1000 A being the practical lower limit in size. Election beam microdefinition technology differs quite drastically from photoresist technology in that in photoresist technology, designers make large patterns out of a sheet of red plastic with the definition of the different elements of the pattern resulting from the cutting out of certain areas. The large plastic sheet is then photographed and reduced a number of times to bring the pattern down to the correct size so that the pattern can be transferred by light to the photoresist. In production, this procedure usually takes from one to two weeks from the design stage to the patterned resist.
In the case of electron beam technology, an electron beam is scanned across the resist itself to form the desired pattern. The electron beam is controlled by a computer which has been fed the coordinates of the pattern as previously determined by a designer. Thus, the use of the electron beam has eliminated all the time lost in preparing the reduction photography required to from a patterned photoresist. However, due to the pattern in the electron beam resist resulting from the scan of a very narrow electron beam, the reaction time of the resist to the electron beam is the time drawback to the production use of electron beam resists.
Obviously, then, in addition to the characteristics required of a good photoresist, such as: good adhesion to many materials, good etch resistance to conventional etches, solubility in desired solvents, and thermostability, an electron resist must react to the electron beam irradiation fast enough to allow a reasonable scan time of the electron beam. In order to bring electron beam technology into production status, resists composed of thin polymer films that are capable of retaining an image of one micron or less at very high scanning speeds of the electron beam are required.
A number of approaches have been taken in the past to develop practical electron beam resists. The first approach and one that proved to be the least successful was the use of convention photoresists, which are also polymers. Although capable of being exposed at relative high scan rates, such exhibit line widths, i.e., resolutions, greater than one micron in width.
The most widely used electron beam resist today is polymethyl methacrylate (PMMA), a positive resist. PMMA is characterized by excellent resolution and line width characteristics and by good processability. However, PMMA requires a relatively slow exposure rate, of approximately 5 .times. 10.sup.-5 coulombs/cm.sup.2, and has the inability to withstand strong oxidizing acids and base etches. A good electron beam resist must react at least ten times faster than PMMA and must withstand strong oxidizing acids and base etches.
A random copolymer of styrene-butadiene has also been proposed. See U.S. Pat. No. 3,794,510.
Therefore, an object of this invention is to provide a method of forming a negative electron beam resist which can be scanned by a beam of electrons at a rate greater than 5 .times. 10.sup.-5 coulombs/cm.sup.2.
Another object of this invention is to provide a method of forming a negative electron beam resist that can withstand strong oxidizing acids and base etches.
Another object of this invention is to provide a method of forming a negative electron beam resist having good adhesion to many support materials.
Another object of this invention is to provide a method of forming an improved styrene-diene negative electron beam resist.
Another object of this invention is to provide a method of forming a negative electron beam resist that is thermostable.
A further object of this invention is to provide a negative electron beam resist that can withstand strong oxidizing acids and base etches; has good adhesion to many support materials; and is soluble in many common solvents.